Polymerization apparatus



Oct. 18, 1966 F. J. HOWE 3,279,895

POLYMERIZATION APPARATUS Filed Jan. 14, 1964 INVENTOR FREDERICK J. HOWEUnited States Patent 3,279,895 POLYMERIZATION APPARATUS Frederick J.Howe, Pensacola, Fla, assignor to Monsanto Company, a corporation ofDelaware Filed Jan. 14, 1964, Ser. No. 337,658 6 Claims. (Cl. 23-285)The present invention concerns continuous polymerization apparatus whichis specially constructed to provideuniform retention time, controlledmixing, and excellent film formation.

One of the principal steps encountered in the production of polymericmaterials by melt polymerization is the finishing step during which thepolymerization reaction is carried to its completion. In continuousprocesses such as those used for nylon and polyethylene terepthalate,this final polymerization step is normally carried out in heatedcylindrical horizontal vessels known as finishers.

It is desirable that these finishers provide substantiallycompartmentalized (flow with a minimum of mixing in the axial direction)of the polymer being treated since polymers are subject to degradationif maintained at elevated temperatures for extended periods of time.Considerable mixing of the polymer within each compartmen is desirablein order to provide a uniform output product and to facilitate contactbetween reactive molecules so that the desired reaction may go forward.It is thus necessary to retain the polymer in the finisher forsufiicient time to complete the reaction, but equally necessary toprevent any particular group of molecules from being retained muchlonger or shorter than the average retention time for all molecules.

The removal of gaseous or vaporous reaction products from high viscositymelts is usually difficult to achieve in a reasonable length of time.Such removal is greatly facilitated by forming the polymer into a filmto increase its surface area.

Prior art finishers have used both solid and perforated continuousscrews for forwarding the polymer. The solid screw can generally bedesigned to provide substantially compartmentalized flow, but mostpracticable designs require operation at slow speed, such as from 1 to 3revolutions per minute. These slow speeds produce poor mixing within thecompartments and low film-forming efliciencies. Only limited improvementcan be realized with perforated screw flights, because if theperforations are sufficiently large to reduce the pumping efficiency andallow higher screw speeds with consequent improvement in mixing withineach compartment, backward and forward mixnig through the perforationsbetween adjacent compartments becomes objectionable.

An alternative approach involves the use of discontinuous screwscontaining solid and/ or perforated discs, screw flight segments, andthe like. This permits the use of higher screw speeds but thecharacteristic flow patterns created by this arrangement generateobjectionable back and forward mixing. Thus this approach likewisecauses the residence time of many molecules to deviate substantiallyfrom the average, and satisfactory compartmentalized flow is notrealized.

By a recent invention of J. E. Tate and G. L. Whitesell, disclosed in aUS. patent application filed concurrently herewith and identified asSerial No. 337,608, filed January 14, 1964, these disadvantages of theprior art are overcome by constructing the polymer-forwarding apparatusin the form of a radially perforated horizontal rotating cylinder,through which the polymer flows axially. According to the presentinvention, the perforated member forming the cylinder is oriented so asto impart substantially greater torsional rigidity to the cylinder, aswill be more fully explained below.

Accordingly, a primary object of the invention is to provide a shaftlesspolymerization apparatus having increased rigidity.

A further object is to provide polymerization apparatus of the abovecharacter which produces excellent mixing of molecules having the sameresidence times.

A further object is to provide polymerization apparatus of the abovecharacter which has excellent film formation properties, thus providingfor eflicient removal of the gaseous reaction products.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of part-s which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing, in which:

FIGURE 1 is a side elevation view partly broken away, of an exemplarypolymerization apparatus according to the present invention; and FIGURE2 is a schematic perspective view, partially broken away, of the polymerhandling apparatus.

Referring now generally to FIGURE 1, there is illustrated a finisher 20of the general type disclosed in the Tate et al. application 337,608,wherein the perforated member is oriented according to the presentinvention. Finisher 20 comprises a general cylindrical finishing chamber22 surrounded by a heating jacket 24, and includes a novel polymerhandling and conveying assembly 26 disposed within chamber 22. Partiallypolymerized material of low molecular weight is fed into the left end ofchamber 22 as viewed in FIGURE 1 through an input supply line 28, whilethe finished high molecular weight polymer is removed through an outputline 30 at the right end of chamber 22 as viewed in FIGURE 1. An exhaustline 32 communicates with the upper portion of chamber 22 in order toremove the gaseous reaction products.

The novel polymer handling and conveying assembly 26 is best illustratedin FIGURE 2, to which reference is now made. A pair of stub shafts 34and 36 are aligned on the axis of assembly 26, and support the remainderof the generally cage-like assembly 26. Stub shafts 34 and 36 haverigidly mounted on their opposed ends screw segments or flights 38 and40, respectively. A plurality of horizontal rods 42 connect flights 38and 40, to form a generally cylindrical frame. Rods 42 may be weldedinto mating notches in the periphery of flights 38 and 40, and should beparallel to the axis of shafts 34 and 36. An annular radially-perforatedcylindrical member 44, illustrated in the form of a wire mesh, ismounted on the periphery of flights 38 and 40, and is preferably weldedto and supported by the several rods 42. Additional flights 46 may beadded to assembly 26 as necessary. Short cores 48 may be welded to thecenters of flights 46 to provide stiffness. Preferably these additionalflights have their peripheries notched to mate with the several rods 42,and are welded in place.

Mesh 44 should have perforations or apertures there- Q through havingminimum dimensions no less than A; inch and no greater than 1 /2 inch.The optimum minimum dimension is about /2 inch. Advantageously, asillustrated, the dimensions of the apertures may increase toward the endwhere the polymer is most viscous, i.e., the apertures nearest shaft 36may be larger than those near shaft 34. This increase in aperture sizemay be simply provided by removing selected segments of the Wire mesh.

'A major feature of assembly 26 is the absence of a conventional centralshaft, which would normally be integral with stub shafts 34 and 36. Whena central shaft is used, the viscous polymer tends to adhere to androtate with the shaft, causing irregular flow and substantial deviationsfrom the desired uniform residence times. The assembly 26 according tothe Tate et al. invention replaces such a central shaft with the twostub shafts which support the remainder of the cage-like assembly.

According to the present invention, wire mesh 44- is applied to theframework with each individual wire thereof arranged in a helix aboutthe framework, rather than with some of the wires running parallel tothe axis of assembly 26 and the remainder forming circles about theframework. Each intersection of each Wire with the several rods 42 orthe flights 38, 40, or 46, is preferably welded to impart the maximumdegree of rigidity to the assembly. Maximum rigidity is produced whenthe helix angle formed by one set of the wires is between and 60,although some increase in rigidity is produced by any angle greater thanzero. A helix angle of 45 is convenient and is preferred. When the meshis oriented according to the present invention, it lends substantialtorsional rigidity to assembly 26, to compensate for the absence of acontinuous central support shaft. This added torsional rigidity isparticularly significant when only one of stub shafts 34 and 36 isdriven.

Assembly 26 is installed in chamber 22, the lower surface of \whichshould fairly closely conform to the cylindrical shape of the perforatedcylinder formed by mesh 44. Stub shaft 34 extends through the intake endwall of chamber 22 through a suitable seal and bearing assembly 52,while stub shaft 36 is journalled in another bearing 54 near outlet line30. Shaft 34- is driven by a suitable motor (not illustrated) which maybe of any desired type. For optimum operation, the polymer level inchamber 22 should be approximately along the axis of assembly 26,although satisfactory operation is maintained if the levelis withinabout /6 the diameter of assembly 26 above or below this level.

In the operation of the apparatus as thus described, the relatively lowmolecular weight polymer fed through line 28 (see FIGURE 1) is conveyedby assembly 26 slowly through chamber 22 until it is finally dischargedthrough line 30 as a high molecular weight polymer. As the polymerpasses through chamber 22, most of the polymer will be contained withinthe cylinder defined by mesh 44. As assembly 26 is rotated by shaft 34,polymer will bridge the apertures between adjacent strands of the wiremesh and will be carried up until it falls from the mesh and back intothe interior of assembly 26. The polymer in so falling away from thewires constituting the mesh will form films resembling rectangularhoneycombs and having a great surface area. This facilitates greatlyremoval of the reaction gaseous by-products, thus promoting thepolymerization reaction.

It should be particularly noted that since the mesh moves in a circularpath with its wall parallel to the direction of polymer flow, there areessentially no forces which tend to mix portions of the polymer whichhave been in chamber 22 for different times (i.e., there is es sentiallyno mixing in a direction parallel to the axis of assembly 26), whilethere is very effective mixing of polymer in all planes perpendicular tothe axis of cylinder 26 as well as formation of films having greatlyincreased surface area.

As an example of a finisher particularly adapted for rapidpolymerization of polyethylene terephthalate, assembly 26 hadapproximately a 7%. inch diameter, with about inch clearance between itslower half and the wall of chamber 22. Approximately two inchesclearance was provided between the top of assembly 26 and chamber 22.Flights 38 and 40 were about nine feet apart with intermediate flights46 at one foot intervals, although a lesser number of intermediateflights could be used if desired. Each flight had a one inch pitch,except fiight 40, which was in the form of a four-bladed propeller witha four inch pitch. The three feet of mesh nearest flight 38 was formedof /2 x /2 inch wire cloth, the central three feet was formed /2 x 1inch wire cloth (formed by removing portions of /2 x /2 inch wirecloth), while the three feet nearest flight 40 was formed of l x 1 inchwire cloth. Assembly 26 was rotated at between about three and elevenr.p.m., providing excellent radial mixing and rapid polymerization dueto the large film surface area created, together with a minimum ofmixing of polymer in the axial direction. It is to be understood thatthese specific parameters are merely illustrative and are not criticalto the invention.

It may be seen from the above description and the accompanying drawingsthat the cage assembly 26 according to the present invention providessubstantially increased torsional rigidity as compared to similarconstructions wherein the wires or other filamentary members are notdisposed helically on the cage periphery. When the helix angle isselected to be within the preferred range indicated above, maximumrigidity of the cage assembly is provided. This feature is especiallysignificant when the cage assembly 26 is driven only from one end, andwhen it is of shaftless construction. The apparatus as disclosed issimple and eflicient, and is well adapted for use in polymerizingvarious polymers such as nylon or polyethylene terephthalate.

I claim:

1. A shaftless cage assembly for handling and conveying polymer materialin a polymer finisher, said cage assembly comprising, in combination:

(a) a plurality of spaced axially aligned fiights for axially conveyingsaid polymer material,

(b) a plurality of substantially circumferentially ar ranged spaced rodsconnecting said flights and forming therewith an elongated cylindricalframe,

(c) a plurality of elongated filamentary members helically wound in afirst direction on and secured to said frame,

(d) and support members secured at each end of said elongatedcylindrical frame to support and rotatably drive the latter.

2. The shaftless cage assembly defined in claim 1 wherein saidfilamentary members form a helical angle of between 30 and 3. Theshaftless cage assembly defined in claim 1 wherein said filamentarymembers form a helical angle of about 45.

4. A shaftless cage assembly for handling and conveying polymer materialin a polymer finisher, said cage assembly comprising, in combination:

(a) a plurality of spaced axially aligned flights for axially conveyingsaid polymer material,

(b) a plurality of substantially circumferentially arranged spaced rodsconnecting said fiights and forming therewith an elongated cylindricalframe,

(c) a first plurality of elongated filamentary members helically woundin a first direction on and secured to said frame,

(d) a second plurality of elongated filamentary members oppositelyhelically wound on and secured to said frame, whereby said first andsaid second pluralities of filamentary members form an open meshcylindrical cage,

(e) and support members secured at each end of said 3,279,895 5 6elongated cylindrical frame to support and rotatably I References Citedby the Examiner drive the latter- UNITED STATES PATENTS 5. The shaftlesscage assembly defined in :clalm 4 where- 1 in at least one of saidpluralities of filamentary members 3,046,099 7/1962 Wlney 23*285 formshelical angles of between 30 and 60. 5

6. The shaftless cage assembly defined in claim 5 MORRIS WOLK PnmaryExammer' wherein at least one of said pluralities of filamentary JAMESH. TAYMAN, JR., Assistant Examiner. members forms a helical angle ofabout 45.

1. A SHAFTLESS CAGE ASSEMBLY FOR HANDLING AND CONVEYING POLYMER MATERIALIN A POLYMER FINISHER, SAID CAGE ASSEMBLY COMPRISING, IN COMBINATION:(A) A PLURALITY OF SPACED AXIALLY ALIGNED FIGHTS FOR AXIALLY CONVEYINGSAID POLYMER MATERIAL, (B) A PLURALITY OF SUBSTANTIALLYCIRCUMFERENTIALLY ARRANGED SPACED RODS CONNECTING SAID FIGHTS ANDFORMING THEREWITH AN ELONGATED CYLINDRICAL FRAME, (C) A PLURALITY OFELONGATED FILAMENTARY MEMBERS HELICALLY WOUND IN A FIRST DIRECTION ONAND SECURED TO SAID FRAME, (D) AND SUPPORT MEMBERS SECURED AT EACH ENDOF SAID ELONGATED CYLINDRICAL FRAME TO SUPPORT AND ROTATABLY DRIVE THELATTER.